Though exclusively acoustic methods for wind profiling and the like have a long history, Coulter & Kallistratova in their 1999 review article “The Role Acoustic Sounding in a High-Technology Era” [Meteorol. Atmos. Phys. 71, 3–19] show that their performance has not been satisfactory, largely due to an Inability to achieve an adequate signal-to-noise ratio [s/n].
In recent years, radar DSP (digital signal processing) techniques have been applied to the sodar to achieve improved s/n. In particular, pulse-compression techniques have been used, in which the echoes from a phase or frequency coded acoustic pulse are processed with matched filters using Fourier transforms to give the range resolution normally associated with a shorter pulse with a much higher peak power. Such coded pulses are said to have ‘pulse-compression’ waveforms or to be ‘pulse coded’. [For short, pulses of this type will be called ‘chirps’ herein.] In an article entitled: “Use of Coded Waveforms for SODAR Systems” [Meteorol. Atomos. Phys. 71, 15–23 (1999)], S G Bradley recently reviewed, with simulations, the use of radar pulse compression techniques to improve amplitude discrimination in sodar. Examples of the use of pulse compression techniques in radar can be found in U.S. Pat. No. 6,208,285 to Burkhardt, U.S. Pat. No. 6,087,981 to Normat et al, and U.S. Pat. No. 6,040,898 to Mroski et al. Despite the application of such sophisticated techniques to sodar, a review by Crescenti entitled, “The Degradation of Doppler Sodar Performance Due to Noise” [Crescenti, G. H., 1998, Atmospheric Environment, 32, 1499–1509], found that severe problems remained even at modest ranges of 1500 m.
In our copending International patent application PCT/AU01/00247 [WO 01/67132] we disclosed sodar systems using long chirps of tens of seconds in duration, in which systems echoes from each chirps were detected while the transmission of the chirp was still continuing. [Such systems can be termed ‘listen-while-sending’ systems and are to be contrasted with the conventional ‘send-then-listen’ systems characteristic of both prior radar and sodar art.] Fourier-based pulse-compression techniques were used in our prior application to extract the desired faint echo signals from interference, which includes the direct signal that Is received directly from transmitter during transmission. In that copending application we indicated that linear acoustic chirps in the frequency range of 500 to 5000 Hz were suitable. We also disclosed the use of over-sampling; that is, the use of sampling rates well in excess of the Nyquist frequencies for chirp tones.
While the combination of the above characteristics of the system of our copending application served to greatly improve s/n with respect to the art, the use of long chirps and listening-while-sending created special challenges relating to interference removal so as to reveal fine-scale discontinuities in the lower atmosphere. There are three primary components of interference—the direct signal, ambient noise and signal clutter. Ambient is of three types: ‘noise spikes’ caused by short loud noises such as fire-crackers or gun-shots, cars back-firing and the like; background noise such as traffic hiss and rumble; and acoustic echoes of the transmitted chirp returned from fixtures such as nearby buildings. Clutter refers to echoes returned from moving objects, such as flocks of birds or waving trees, which are not of interest. It will be appreciated that, unlike conventional short-pulse send-then-listen radar and sodar, the long listening times that we prefer mean that a lot of noise is collected.
While monostatic sodar systems can be made compact and conveniently portable, especially where the transmitter and receiver are mounted on the same dish or mechanical structure, direct signal interference is a severe problem in listen-while-sending systems because of its large amplitude with respect to echoes. Also, with monostatic systems it is most difficult to separate returned Doppler components due to horizontal and vertical wind speed. These problems are much less intrusive in bi-static systems where the receiver is well removed from the transmitter thereby greatly reducing the direct signal and, because of the use of small angles of reflection/refraction, Doppler signals due to horizontal wind are naturally favored over those due to vertical windspeed. However, such bistatic systems suffer badly from propagation losses due to the much longer signal path compared with monostatic systems that are pointed substantially vertically.
U.S. Pat. No. 2,507,121 to Sivian [1950] disclosed a short-pulse, send-then-listen, monostatic acoustic system for detecting the height of atmospheric discontinuities. In the embodiment of most interest here, two receivers (microphones) were used, one being shielded from returned echoes and the other not. The two receivers were connected so that their outputs were opposed and the net signal was displayed on an oscilloscope. In the event of a normally returned echo, a pip is displayed because only the second receiver detects a signal. However, in the event of a local gunshot during the listening time, both receivers detect the same signal and no pip is displayed.
U.S. Pat. No. 3,675,191 to McAllister [1972] disclosed a short-pulse, send-then-listen, monostatic, sodar system using four adjacent arrays of acoustic transducers capable of being used as speakers and microphones, the arrays being aligned with the cardinal points of the compass and being shielded from one another, except at their upper faces. Short acoustic pulses were transmitted vertically upwards and the relative timing of the returned echoes at each of the four arrays gave the height and bearing of wind layers. [It might be noted that the physics of acoustic sounding was well documented in 1969 by McAllister and others in “Acoustic Sounding—A New Approach to the Study of Atmospheric Structure” in Proc. IEEE Vol. 57, 579–587.] A similar system was disclosed by U.S. Pat. No. 4,558,594 to Balser where an acoustic phased array was used that was capable of directing successive pulses in different directions, the echoes from one pulse being detected by the array before the next was transmitted. U.S. Pat. No. 5,521,883 to Fage et al used a phased array to send pulses of different frequencies in different directions and then listen for all echoes simultaneously, thereby decreasing the cycle time. The typical angle of elevation for pulse transmission in the latter systems was between 20 and 30 degrees. The relatively low elevation angles enhanced Doppler components in the returned echoes due to horizontal rather than vertical wind speed but suffered from serious propagation attenuation.
In U.S. Pat. No. 6,097,669 Jordan teaches the transmission of a high-powered short-duration acoustic pulse consisting of a string of concatenated wavelets in a send-then-listen system. The echoes are sampled and range-gated. An amplitude peak surrounding the frequency of the transmitted sound is observed for each range gate and used to derive wind velocity at the altitude concerned using wavelet coefficients and inverse wavelet transforms. This patent makes reference to a publication by Jordan et al entitled, Removing Ground and Intermittent Clutter Contamination from Wind Profiler Signals using Wavelet Tansforms [Mar. Vol. 14 Journal of Atmospheric and Oceanic Technology, 1280–1297] that relates to radar rather than sodar methods. Similarly, earlier disclosures by Jordan, as in U.S. Pat. No. 5,592,171, relate to wavelet methods for use in [send-then-listen] radar wind profiling where discrimination against clutter and variable noise is claimed. In U.S. Pat. No. 5,686,919 Jordan disclosed somewhat similar polynomial techniques for removing broadband clutter from radar send-then-listen systems. Such clutter removal methods are, however, quite unsuited for systems using long chirped pulses and listening while sending.